Methods of transmitting and receiving communications signals over fiber optic networks are known. Wavelength division multiplexing (WDM) techniques are also known in the context of optical communications systems. Such techniques have been applied to a variety of communications networks, including metropolitan local area networks, regional wide area networks and long distance communications networks. In fact wavelength division multiplexing is a standard technique used today to increase aggregate capacity in fiber optic communications systems.
The prior use of wavelength division multiplexing techniques in fiber optic networks has generally limited the spectrum utilized for the transmission signal to a small fraction of the occupied bandwidth. This approach has been effective to increase capacity by combining multiple digital signal streams on a single fiber by using a different wavelength (channel) as the carrier for each of the multiple data streams. However as the number of separate wavelengths increases, there is much economic benefit to be gained from a more efficient utilization of spectrum.
The current industry trend is to increase spectral efficiency by reducing the frequency spacing between optical carriers in the multiplexed transmission. It is not uncommon to find optical channels carrying 10 Gbps transmissions on frequency grids spaced at 100 GHz and 50 GHz frequency grids are starting to be deployed. Even at the 50 GHz frequency separation for the 10 Gbps signal, the spectral efficiency is only 0.2 bps/Hz (20%). Nevertheless it is feasible that wavelength division multiplexing technology can yield spectral efficiencies approaching 100%.
The need to improve spectral efficiency is driven by economic as well as technological reasons. This is because in general any given component technology operates over a limited bandwidth. Increased spectral efficiency optimizes the effective utilization of such limited bandwidth.
A large spectral separation between neighboring optical channels in a WDM system allows for a great deal of error in absolute frequency allocation. Typically a relatively low spectral efficiency can accommodate an optical filter that provides a flat passband over a bandwidth much greater than that of the signal. Thus the system can accommodate substantial errors in the center frequency of both laser and filter frequencies.
However as the frequency grid is made narrower, the absolute accuracy to which laser and filter center frequencies must be established and maintained becomes increasingly important to achieve reliable system performance. Furthermore, increases in spectral efficiency are fundamentally limited when the Fourier bandwidth of the signal exceeds the flat portion of the filter passband width. As these limits are reached, transmission impairments caused by frequency errors result. Therefore the accuracy and stability of the spectral components become critical performance parameters in WDM systems with high spectral efficiency.
While the use of multiple optical signals for the transmission of information through a single waveguide is an effective means to increase bandwidth, the optical output frequency of a laser is sensitive to any of a number of factors (e.g. age, temperature, etc.). Hence it is well known the laser frequency may change during its operating lifetime. Alternatively, the laser itself may fail. If the optical carrier frequency changes by too much (or if the laser fails), downstream detecting and decoding can no longer function properly in a WDM system. Therefore means to maintain laser frequency stability during the operational lifetime are required to enable deployment of closely spaced WDM systems.
Furthermore, as frequency spacing between individual optical carriers is reduced, the need arises to accurately determine the laser transmission frequency for each optical signal. The ultra-high frequency of individual optical carriers makes such frequency measurement difficult. Nevertheless, it is important to improve spectral efficiency of WDM transmissions in order to achieve the most favorable economics while increasing the bandwidth capacity of optical communications systems. Thus there is a need for a method of measuring and stabilizing the frequency of optical carriers in fiber optic systems containing large numbers of optical wavelengths.